1. Field of the Invention
The present invention relates to a portable telephone and, in particular, it relates to a portable telephone capable of minimizing absorption of an electromagnetic energy radiated therefrom by a human body when in use thereof.
2. Description of the Related Art
FIG. 6 is a schematic diagram showing a conventional portable telephone. As shown in FIG. 6, a portable telephone 201 is used being held in a close proximity to a head (HEAD) of a user. The portable telephone 201 is comprised of an antenna 31, a feeder 32, a printed board 33, a shield case 34, semiconductor integrated circuits 35A and 35B, a microphone 36, a keypad 37, a liquid crystal display 38, a speaker 39 and an insulated housing 40.
The antenna 31 for use of signal transmission and reception is allowed to project upward from an upper edge of the housing 40 for transmitting electromagnetic waves to a base station in a transmission mode and receiving electromagnetic waves from the base station in a reception mode. The antenna 31 is a retractable antenna capable of retracting in a longitudinal direction of the housing 40.
On one surface of the printed board 33, there is mounted a semiconductor integrated circuit 35A constructed including transistors, capacitors, resistors and so on. Also, on the other surface of the printed board 33, there is mounted a semiconductor integrated circuit 35B constructed likewise.
The printed board 33 and the semiconductor integrated circuits 35A and 35B in combination constitute a printed circuit board 35. On this printed circuit board 35, there are mounted a switch circuit, a control circuit, a transmitting circuit and a receiving circuit. Between the antenna 31 and the transmitting and the receiving circuits are connected via the switch circuit and the feeder section 32.
The microphone 36 constitutes a transmitter (telephone microphone), and the speaker 39 constitutes a receiver (telephone earphone). The housing 40 is provided with an opening 36H for the microphone 36, an opening 39H for the speaker 39, a window 38W for the liquid crystal display section 38 and the like. The housing 40 accommodates the printed circuit board 35, the shield case 34, the microphone 36 and the speaker 39.
A voice or sound uttered by the user is supplied to the microphone 36 through the opening 36H. The microphone to which the sound or the like uttered by the user is input generates a first audio signal corresponding to the sound uttered by the user. The printed circuit board 35 converts the first audio signal to a first radio signal. The first radio signal is transmitted from the antenna 31 to the base station. The base station sends a second radio signal to the portable telephone 201.
The printed circuit board 35 converts the second radio signal having been input to the antenna 31 into a second audio signal to be supplied to the speaker 39. The speaker 39 outputs a sound corresponding to the second audio signal to be supplied to the user.
Circuits such as the printed circuit board 35, the microphone 36, the speaker 39, the liquid crystal display 38 and so on are supplied power from a drive source (a battery, not shown) within the portable telephone 201.
The user uses the keypad 37, which has ten key buttons and the like, for entering information such as telephone numbers or the like. The liquid crystal display 38 displays information such as characters, images and the like.
The shield case 34 which is connected to a ground layer of the printed circuit board 35 is arranged to surround the printed circuit board 35 in order to suppress an electromagnetic interference between the printed circuit board 35 and the antenna 31. The shield case 34 is formed using an electrical conducting material such as a metal, or a plastic material which is coated with a metal or the like on the surface thereof.
In recent years, guidelines for an electromagnetic energy partial-body absorption directed to the portable telephone have been set up in the USA, Europe and Japan, consecutively. As a quantity of evaluation for the partial-body absorption, a power absorbed in a unit mass, that is, a specific absorption rate (SAR) is used. Assuming that an electric field invaded into a biological tissue is E, a conductivity of the biological tissue is "sgr" and its density is xcfx81, the SAR can be expressed by the following equation (1).
SAR="sgr"E2/2xcfx81xe2x80x83xe2x80x83(1)
For example, in the partial-body absorption guideline in the USA, that is, FCC""s guideline, it is specified that a peak value in an average SAR per gram tissue does not exceed 1.6 W/kg. Also, in the partial-body absorption guidelines in Europe and Japan, it is specified that a peak value in an average SAR per 10 gram-tissue does not exceed 2 W/kg.
It is described in the paper titled xe2x80x9cReduction of the partial-body SAR and assurance of the communication characteristics by controlling the surface current in the housing of a portable telephonexe2x80x9d published in the Transactions of Japan Society of Applied Magnetism Vol. 23, No. 10, pages 2005-2008, 1999 that the partial-body SAR can be reduced by attaching a ferrite sheet on the metal housing of the portable telephone.
The SAR tends to increase when a transmission power of the portable telephone becomes greater, and a distance between a radiation source of waves in the portable telephone and the body of the user becomes closer. It may occur that the partial-body SAR of the portable telephone becomes maximum in a part other than the antenna, for example, in the vicinity of the shield case.
This is caused partly due to that a size of the housing is too small compared to the electromagnetic wavelength used in the radio communications, i.e., less than one wavelength thereof.
For example, in contrast to that a wavelength of electromagnetic waves at a frequency of 900 MHz is 330 mm and that at a frequency of 2 GHz is 150 mm, a size of the portable telephone in the longitudinal direction is normally 120 mm or so. The power supplied to the antenna is also supplied to the conductive shield case. Thereby, the shield case, which apparently is not the antenna, functions as a part of the antenna serving as a radiation source of electromagnetic waves.
Another cause that the partial-body SAR becomes maximum not at the antenna but in the vicinity of the shield case is indicated due to that during use of the portable telephone, the shield case comes closer to the head of the user than the antenna. Portable telephones now available on the market have become more compact and thinner sized, in comparison with the portable radio transceiver equipment, which have been in use since before the proliferation of the portable telephones. Therefore, a distance in particular between a conductive portion of the housing and the body of the user is substantially reduced in comparison with that of the portable radio equipment, thereby substantially increasing the partial-body SAR in the vicinity of the shield case thereof.
FIG. 7 shows a schematic diagram of a portable telephone 202 in which an electromagnetic wave absorption layer 41 which contains a magnetic loss material is pasted on its shield case 34. The portable telephone 202 indicated in FIG. 7 has the electromagnetic wave absorption layer 41 formed into a sheet, and contains the magnetic loss material. The other components and parts except for the above layer 41 are the same as those of the portable telephone 201 indicated with reference to FIG. 6, and are depicted with the same numerals and symbols.
In the portable telephone 202 indicated in FIG. 7, as its magnetic loss material for use in the electromagnetic wave absorption layer 41, there are cited, for example, ferrite, Permalloy, Sendust, stainless steel, silicon steel, ferro-amorphous alloys and the like.
Further, in the portable telephone 202, its electromagnetic wave absorption layer 41 is pasted on its shield case 34 via an adhesive layer on the side thereof facing the head H of the user. For example, an ultraviolet curable adhesive or a thermosetting adhesive are used as the adhesive.
In the portable telephone 202 having the above-mentioned construction, in order to reduce its SAR, for example, by approximately 20%, it becomes necessary for the electromagnetic wave absorption layer 41 including the magnetic loss material to have a thickness at least of 2-3 mm. In the portable telephone, however, a more compact-sized and light-weighted design thereof is strongly desired, therefore, respective components and parts of the portable telephone are also desired to be made further thinner and lighter-weighted.
Still further, according to the arrangement indicated in FIG. 7, although its SAR can be reduced by approximately 20% in comparison with an arrangement without the provision of the electromagnetic wave absorption layer 41, it is difficult to reduce its SAR any further.
The present invention has been contemplated to solve the above-mentioned problems associated with the prior art, and therefore, the present invention provides a portable telephone that can reduce more effectively an electromagnetic energy absorption by the human body during its use.
A portable telephone provided according to one aspect of the invention is characterized by comprising: an antenna; a microphone; a transmitting circuit; a receiving circuit; a receiver; a printed circuit board; a shield case; a housing; an electromagnetic wave absorption layer; and an electromagnetic wave reflection layer. The microphone generates a first audio signal corresponding to an input sound. The transmitting circuit modulates the first audio signal from the microphone so as to generate a first radio signal, and transmits the first radio signal via the antenna. The receiving circuit modulates a second radio signal which is modulated from a second audio signal and is received via the antenna, and produces the second audio signal. The receiver outputs sound corresponding to the second audio signal from the receiving circuit. The printed circuit board has the receiving circuit and the transmitting circuit mounted thereon. The shield case surrounds the printed circuit board in order to suppress an electromagnetic interference between the printed circuit board and the antenna, and is electrically conductive at least on its surface. The housing accommodates the microphone, the receiver and the shield case. The electromagnetic wave absorption layer includes a magnetic loss material, and is formed at least in a part between the shield case and the housing. The electromagnetic wave reflection layer contains a magnetic reflection material, and is formed between the housing and the electromagnetic wave absorption layer.
Preferably, the portable telephone according to the invention is characterized in that the above-mentioned electromagnetic wave absorption layer is formed in contact with the shield case adjacent to the receiver, and the above-mentioned electromagnetic wave reflection layer is laminated on the electromagnetic wave absorption layer.
Alternatively, the portable telephone of the invention is characterized preferably in that the above-mentioned electromagnetic reflection layer is formed in contact with the housing in the vicinity of the receiver, and the above-mentioned electromagnetic wave absorption layer is laminated on the electromagnetic wave reflection layer.
The portable telephone according to the invention is characterized in that the above-mentioned electromagnetic wave absorption layer is preferably pasted on the shield case with an adhesive.
Alternatively, the portable telephone according to the invention is characterized in that the above-mentioned electromagnetic wave reflection layer is preferably pasted on the housing in the vicinity of the receiver.
The portable telephone according to the invention is characterized in that the above-mentioned electromagnetic wave absorption layer is preferably comprised of a material having a complex magnetic permeability a real part of which is small while an imaginary part of which is large at frequencies of the above-mentioned the first and the second radio signals, and in that the above-mentioned electromagnetic wave reflection layer is preferably comprised of a material having a complex magnetic permeability a real part of which is large while an imaginary part of which is small at frequencies of the first and the second radio signals.
The portable telephone according to the invention is characterized in that the above-mentioned electromagnetic wave absorption layer thereof preferably includes a layer which is prepared by mixing the above-mentioned magnetic loss material with a synthetic resin and molded. Further, the portable telephone according to the invention is characterized in that the above-mentioned magnetic loss material thereof preferably contains at least one selected from the group consisting of ferrite, Permalloy, Sendust, stainless steel, silicon steel, and ferro-amorphous alloy.
The portable telephone according to the invention is characterized in that the above-mentioned electromagnetic wave reflection layer thereof preferably includes a layer which is prepared by mixing the above-mentioned magnetic reflection material with a synthetic resin and molded. Further, the portable telephone according to the invention is characterized in that the above-mentioned magnetic reflection material thereof preferably contains carbonyl iron. Alternatively, the portable telephone according to the invention is characterized in that the above-mentioned magnetic reflection material preferably contains ferrite.
The portable telephone according to the invention is characterized in that the above-mentioned shield case preferably has a casing made of an insulating material and an electrical conducting layer which is formed on a surface of this casing.
Thereby, it is enabled more effectively to reduce the electromagnetic energy absorption by the human body during use of the portable telephone. Advantageously, according to a lamination film of the electromagnetic wave absorption layer and the electromagnetic wave reflection layer provided in the portable telephone of the invention, the SAR thereof can be reduced substantially and more effectively than in the conventional portable telephone in which only the electromagnetic wave absorption layer made of the magnetic loss material is provided.